Just a couple of comments regarding possible issues with any of the methods:
- As TonyW wrote, RGB 255,255,255 is paper white and the common photographic printers will not lay down ink. The only risk with this method is that if you subsequently apply global edits with curves, exposure, etc. then the value of that area might move from 255,255,255.
- In the case of the transparent layer, it may not get preserved in some formats as jpeg, so you should keep it in psd or tiff.
Regards